An internal gear pump is used as, for example, an oil pump for lubricating a vehicle engine, for an automatic transmission (AT), for a continuously variable transmission (CVT), or for supplying diesel fuel.
In a known type of this internal gear pump, the tooth profile of the inner rotor is formed by utilizing a trochoidal curve. As shown in FIG. 8, a diameter A of a base circle, a diameter B of a rolling circle, an amount e of eccentricity, and a diameter C of a locus circle are first set. Then, the rolling circle rolls along the base circle without slipping, and a trochoidal curve T drawn by a point distant from the center of the rolling circle (by the amount e of eccentricity) is obtained. An envelope of a group of circular arcs obtained when a center C0 of the locus circle C is moved along the trochoidal curve T serves as an inner-rotor curve (tooth profile) TC (see FIG. 2 in Patent Literature 1).
An outer rotor used has one tooth more than the inner rotor 2 (the number of teeth of the inner rotor: n, and the number of teeth of the outer rotor: n+1). The tooth profile of the outer rotor is formed based on a method that uses a locus of a group of tooth-profile curves of the inner rotor 2 obtained based on the above-described method, or is formed based on another known method. For example, the former method that uses a locus of a group of tooth-profile curves of the inner rotor involves revolving the center of the inner rotor by one lap along a circle centered on the center of the outer rotor and having a diameter of (2e+t) (e denoting the amount of eccentricity between the inner rotor 2 and the outer rotor 3 and t denoting a tip clearance between the inner rotor 2 and the outer rotor 3 at a theoretical eccentric position), and rotating the inner rotor 2 (1/n) times during the revolution. As the result of the revolution and the rotation of the inner rotor 2, an envelope of a group of inner-rotor tooth-profile curves obtained when the inner rotor 2 rotates n times is drawn, and the envelope serves as the tooth profile of the outer rotor 3 (see FIGS. 3 to 5 in Patent Literature 1, and paragraph [0044] and FIG. 9 in Patent Literature 2).
A pump rotor is formed by combining the inner rotor 2 and the outer rotor 3 manufactured in this manner and disposing these rotors eccentrically relative to each other. This pump rotor is accommodated within a rotor chamber of a housing having an intake port and a discharge port, whereby an internal gear pump is formed (see FIG. 1 in the present application, and paragraph [0048] and FIG. 10 in Patent Literature 2).
In the inner rotor 2 whose tooth profile is formed by utilizing the trochoidal curve, loops R (FIG. 9(a)) may form at opposite edges of each addendum 2a or cusps s (FIG. 9(b)) may form at the opposite edges of the addendum, depending on the selection such as the diameter A of the base circle. A tooth-profile shape having the aforementioned loops R is not realizable in actuality, and since it is impossible that such loops R be formed in a tooth profile, they become cusps s formed at the opposite edges of the addendum.
When a tooth profile having the cusps s at the opposite edges of each addendum is used for a pump, contact stress (i.e., Hertz stress) at the cusps (edges) s increases and causes abrasion or yielding in these areas, thus leading to a reduction in pump performance as well as an increase in vibration and noise.